This disclosure relates in general to a device which can determine if a hole drilled in a work piece is drilled perpendicularly to the work piece surface and, more particularly, to a device for determining the angularity, i.e. the angular displacement or deviation from normality, if any, of the hole. The angularity is measured with respect to the surface where the hole has its entrance opening in the work piece.
It is well known that in certain applications, especially in the aircraft industry, that it is important to determine if a drilled hole which is formed in a work piece is in fact normal to the work piece. In the aircraft industry, it is even more urgent that certain drilled holes are as straight as they are intended to be. For example, if fasteners are installed in holes which significantly deviate from normality in a given structural component of an aircraft, unwanted stress can develop which may result in partial or total failure of the component. In the airplane manufacturing art, countless holes are drilled in the airplane component surfaces which facilitate assembly of the airplane. When you have a nominal sized hole in an airframe, the hole could be a few thousandths of an inch under the nominal size for an interference fit fastener that ideally would never be removed. Alternatively, the hole could be a few thousandths of an inch over the nominal size for a clearance fit fastener that will most likely be removed at some point, such as a bolt holding on a door or piece of electronic equipment. Further adding to the problem of holes deviating from normality, is the fact that different holes will have different tolerances. Rivet holes are loose tolerance since rivets expand to fill the drilled holes. Also, when drilling holes in an airframe, the drilled hole can be made quicker if the hole is slightly tapered. On any given hole, these tolerances can multiply. For example, the drilled hole may be near the low tolerance where the drill exits the material, and near the high tolerance where the drill enters the material. If the fasteners placed into these holes are even slightly misaligned, the structural integrity of the airplane may be compromised. In fact, it is the general practice in the industry to inspect drilled holes to check if they are in fact normal to the work piece. This process of checking if a drilled hole is normal to a work piece is complicated by the fact that the surfaces of an airplane wing or fuselage are, for the most part, curved.
For example, one method, in the prior art, which is cumbersome and complicated, must first determine the hole center line as part of the process of determining hole normality. The center line is determined by using a pin with a diameter as close as possible to the drilled hole, and inserting that pin into the hole. From the previous discussion above, this method becomes problematic. An operator has to carry around a large collection of pins, each with an incrementally different diameter when compared to the last pin, and manually find a pin that “fits” the drilled hole. No pin will properly fit holes as described above which have a low tolerance at one end and a high tolerance at the other end. The pin that “just fits” may fit the bottom portion of the hole, but is free to tip slightly in the hole since it is making contact at only the lower portion of the hole. The cold working process can also inhibit accurate angularity measurement using pins. The cold worked holes when viewed in cross section are not perfectly rectangular due to the cold working process. The top and bottom of the cold worked hole may be slightly dished or bell-shaped at the top and/or bottom of the hole. Thus the pins inserted into the hole to be measured can wobble hampering the measurement process.